This paper describes a modified version of the IAWPRC model and the techniques used to calibrate it on site. Growth rate of autotrophic bacteria and the inert wastewater fractions were measured by simple methods, both in laboratory pilots and on full scale. Model predictions were compared to full scale results in large plants (> 100 000 P E). Measured autotrophic growth rates ranged from 0.3 to 0.65 1/d, depending on wastewater toxicity. Most model constants compared well with original IAWPRC proposals, and several wastewater types were identified. Parameters to establish a suspended solids balance are given. Different configurations of plants for nitrogen removal were investigated, including conventional recirculation, sludge reaeration (RDN) and step feed alternate zone denitrification.

Experience with biofilters, acquired over many years, has made it possible to control their operating parameters. The goal of this article is to present an assessment of operational biofilters, in particular their air consumptions, their sludge productions, their energy balances as well as the duration of their filtration cycles. Some tests, carried out on medium to large full scale units, are described at the same time as the process results. These experiences in the design and operation of biofilters led to their optimization and make it possible to build very large scale installations. Examples of such plants, now being built or started up, are also presented, treating flows in excess of 50 000 m3/d even for low nitrogen residuals.

Biocarbone aerated biofilters were developed more than ten years ago. Almost one hundred plants worldwide provide complete treatment in very short hydraulic retention time. This allows to construct compact plants in sensitive areas such as inner cities, sea or mountain resorts. The filters can be adapted to high effluent qualities expected by the European directive on wastewater treatment. Full oxidation of ammonia can be achieved in less than two hours. A plant for complete removal of nitrogen has been operated for more than a year in Denmark to meet Europe's most stringent effluent requirements by using an anoxic biofilter in series with the Biocarbone. A new aerated biofilter, turning the Biocarbone principle upside down, is presented : a synthetic floating material allows enhanced performance, simplified backwash and a combination of anoxic and aerobic zones in one reactor. Effluent quality of less than 10 mg/l for all components (BOD, SS, TN) was demonstrated in a one-year long on-site test in Denmark and the first facility for 60 000 population equivalents is under startup. Full scale results of the upflow floating filter, Biostyr, are presented for nitrogen and phosphorus removal according to the European directive.